1. Field of the Invention
The present invention relates to a bearing assembly and, more particularly, to a self-lubricating bearing assembly having a debris removal system that improves the performance and/or life of the bearing assembly by removing debris, i.e., wear particles generating during normal bearing operation. The improved bearing assembly finds particular application in a power transmission device which includes trunnion and bearing sets for interconnecting and transferring power from a driving shaft of a power generating unit to a driven shaft of an operating unit and will be described with particular reference thereto. However, it is to be appreciated that the invention may relate to other similar environments and applications.
2. Description of the Prior Art
The mechanical transmission of power normally requires a transfer device which couples a power generating unit (source) to an operating unit in order to perform a mechanical function. The transfer device simultaneously couples the two units and transfers power between them. Often, shafts are used in power transmission assemblies and are joined by the transfer device for rotational and/or reciprocating energy transfer. This arrangement is often referred to as a “drive shaft” and the transfer device within the drive shaft arrangement is commonly called a “universal joint” where four (4) trunnions are utilized and a “tripot” where three (3) trunnions are used. Sometimes, however, the name “universal joint” is used more broadly to refer to any power transfer device regardless of the number of trunnions. A three trunnion tripot can also be referred to as a “constant velocity” joint. A typical drive shaft arrangement will normally utilize two universal joints whereas a “drive line” may use in excess of two universal joints and shafts. Many conventional passenger vehicles employ one drive shaft arrangement having two universal joints whereas many four-wheel drive vehicles and industrial/utility vehicles often employ two or more drive shaft arrangements or drive lines.
Universal joints function to (a) transmit a high starting or high stopping torque, including a reversal of direction of rotation, (b) maintain a continuous transfer of power at either constant or varying drive shaft revolutions, and (c) maintain a maximum power transfer between the power source and the operating unit or units during all relative drive line angles and length variations and vibrations. A common universal joint in use today is that which is sometimes referred to as the Hooke universal joint. In the Hooke joint, the driving and driven shafts are each provided with a yoke, the respective yokes are interconnected by a cross comprised of trunnion-bearing sets. A plurality of transfer surfaces, each essentially cylindrical in shape, are ground at the respective trunnions of the cross. Each trunnion transfer surface is adapted to receive a bearing member or cup which is provided internally with a plurality of needles. Crosses with four (4) transfer surfaces (trunnions) are in widespread use in rear wheel drive (RWD) and industrial vehicle drive shafts. They are also in widespread use in four wheel drive (4WD) vehicles. Tripots with three (3) transfer surfaces (or trunnions) are in widespread use in front wheel drive (FWD) and all-wheel drive (AWD) vehicles.
By introducing needles (cylindrical rollers) that roll, the transfer surfaces are separated from bearing surfaces of the bearing cups to avoid direct contact, greatly improving the relative movement of the transfer and bearing surfaces. The needles serve the function of transferring and maintaining the transmission of torque forces between the bearing cup and the trunnion. However, the universal joint needles are subjected to unusually high pressures which are transferred to the bearing cup surfaces at the area of their contact. The torque pressures often encountered in power transmission devices of conventional vehicles are normally in the 200 to 500 foot-pound range. These torque pressures translate into extremely high psi (pounds per square inch) pressures at the contacting surface. For needle rollers, the pressures range between 200,000 to 2,000,000 psi because of the small contact area (essentially a line). The pressures are orders of magnitude greater for ball bearings which essentially support the torque loads at a point area.
There are several drawbacks associated with the use of needles in bearing assemblies, particularly bearing assemblies used in drive shaft arrangements. One drawback is that, unless periodically lubricated, the bearing assembly will wear and deteriorate so significantly that the bearing member and/or the member (such as a trunnion) having the load transfer surface will require replacement. In a universal joint, this would require replacement of the entire universal joint kit including the bearing cup and the trunnion. Such wear is caused by the normal rolling action between the bearing and the load transfer surface. The relative movement or rolling action causes friction and heat which dries out the lubricant thereby causing the generation of deleterious debris (wear particles), typically 25-50 microns in diameter. As wear continues, the quantity and size of the debris increases. The continuously increasing quantity and size of debris can lodge at the needles causing them to slide rather than roll. This results in the creation of grooves called brinnels, which further restrict the rolling and increase friction, generating more and larger debris. Lack of proper lubrication maintenance accelerates the generation of wear debris. Thus, frequent lubrication and possibly bearing assembly replacement can be required in bearing assemblies employing needles. Frequent lubrication and periodic bearing replacement add an undesirable maintenance cost to the needle using bearing assembly that is necessary to ensure acceptable performance.
Another drawback is that when the bearing assembly with needles is subjected to sudden impacts, the needles transmit extremely high pressure shocks to the bearing and the member having the load transfer surfaces. In the case of a universal joint, sudden impacts to the bearing assembly can be introduced by road pot-holes or off-road vehicle usage and the high pressure shocks are transmitted to the bearing cup and trunnion surfaces. The needles, due to their small contact area (essentially a line), amplify the pressure transmitted which frequently results in a breakdown of the bearing surfaces causing generation of deleterious debris commonly referred to as “galls”. These galls are initially small in size, typically 50-75 micron in diameter. However, they, like the debris causing brinneling, increase in quantity and grow larger over time. In the case of debris that causes brinneling or gall-type debris, once the debris or wear particles within a bearing assembly are larger than about 100 microns, which indicates that wear is progressing rapidly, the bearing assembly should be replaced to avoid complete bearing failure. Should the wear particles exceed about 150 microns in size, bearing failure in the form of surface seizure, is likely to be imminent.
Yet another drawback related to the use of needles in bearing assemblies is the cost factor in manufacturing the needles as additional components of the bearing assemblies and the cost of assembling the needles in a bearing assembly. For example, in the manufacture of universal joints, bearing assemblies having needles are more expensive due to the manufacturing cost of the needles, the cost of assembling the needles in the trunnion and bearing sets of the universal joint and the cost of the required assembly equipment.
Still another drawback of using needles in bearing assemblies is related to the means for enabling periodic lubrication of the bearing assembly. Often, lubricant is used in the area between or adjacent the bearing surfaces and the load transfer surface of the member surrounded by the bearing to provide lubrication as the bearing rolls about the force transfer surface. Heretofore, the means for enabling periodic lubrication of the bearing assembly was a fitting formed as part of the bearing assembly. In a universal joint, the fitting is connected to a channel for each trunnion for purposes of delivering and distributing lubricant forced into the fitting to each of the bearing-trunnion engagement areas. Alternatively, reservoirs drilled into components of the bearing assemblies are sometimes utilized for holding an amount of lubricant suitable for replenishing that which is depleted during normal bearing usage. In either arrangement, the manufacturing costs of the bearing assembly is undesirably increased when such means for enabling periodic lubrication can be incorporated in the bearing assembly.
It is recognized that many other types of bearings operate in non-power transmission situations, e.g., support bearings utilizing roller members, such as needles or balls, supporting a rotating shaft. Another class of support bearings operate without rolling members and are commonly referred to as plain or sliding bearings. Typically, these latter types of bearings rely on sliding movement (as opposed to rolling movement) and either an oil or a non-solid grease to lubricate or facilitate relative movement. One type of non-roller bearing uses oil which is continuously delivered to the surfaces of the bearing by a groove in one of the bearing surfaces. This type of bearing must operate at very high speeds (circa 10,000 rpm) so that the oil can create a hydrostatic film with sufficient force to prevent the load transfer surface and the bearing surfaces from making direct and often metal-to-metal contact. Direct contact leads to friction causing excessive heat and wear, as well as eventual seizure. Fully rotational, constant high speed is required to maintain the film. There is no mechanism for removing wear debris after it is generated in these types of bearings. Moreover, power transmission bearings, including universal joint bearings, are oscillatory at constantly varying speeds which would not permit a film to be generated. Thus, these types of bearings are typically unsuitable for use in power transmission bearing assemblies.
Another type of non-roller prior art bearing relies on various viscosity greases to separate bearing surfaces from load transfer surfaces. The grease is applied to one of the bearing and load transfer surfaces. It has been established by bearing design tribologists that the maximum practical limit of PV (psi-sfm) of this type of bearing is approximately 50,000. PV is a measure of the performance capability of a sliding bearing. P is a measure of pressure on the bearing's surface in pounds per square inch (psi), while V is a measure of the velocity of the bearing's surface in surface feet per minute (sfm). The product of the two, PV, is a indication of the performance capability of the bearing. The PV rating is sensitive to the composition of the materials of the contacting surfaces. Materials chosen for speed (high sfm), V, sacrifice their ability to support load capacity, P. Polymer materials (Polycarbonates, Acetais, Nylon) whose PVs approximate 3,000, with a maximum P of 1,000 psi, fall into this class. Metals permit higher load carrying with PVs ranging from 20,000 to 50,000. Those with low coefficients of friction (i.e., higher lubricity) which include carbon graphite, bronze, tin-bronze, aluminum-bronze, are still limited by the maximum speed permitted, typically under 300 sfm. Conventional bearings fabricated with powdered metals typically operate in the 15,000 to 50,000 PV range. The best performance has been received by PM bearings referred to as porous iron and porous bronze, whose maximum PVs are 30,000 and 50,000, respectively.
Generally, prior art bearings were not designed for removing debris between bearing surfaces and the load transfer surface. Rather, prior art bearings focused on the delivery of liquid lubricants to prevent surfaces from rubbing and generating wear particles. Thus, there is a need for an improved bearing assembly that reduces and/or eliminates the amount and size of deleterious debris or wear particles generated during normal use of the bearing assembly, removes any generated debris from between the bearing surfaces and the load transfer surface, has an increased useful performance life, has increased performance capabilities, and has decreased costs related to the manufacture and maintenance of the bearing assembly.